Method of making granules of alkenylaromatic hydrocarbon polymerrubber compositions with improved extrusion characteristics



United States Patent This invention is concerned with improving the extrusion characteristics of compositions of alkenylaromatic hydrocarbon polymers, such as polystyrene, and natural or synthetic rubber.

There have come into widespread use a number of compositions consisting essentially of a preponderant proportion of an alkenylaromatic hydrocarbon polymer and a small proportion, from 2 to 20 or more percent and commonly from 4 to 8 percent, of natural or synthetic rubber. Such compositions are prepared advantageously by dissolving or suspending natural or synthetic rubber in an alkenylaromatic.hydrocarbon monomer and polymerizing the latter. They may also be prepared by milling the components together. The alkenylaromatic hydrocarbon most commonly employed is styrene, though the vinyltoiuenes may also be. used alone or with styrene, and minor amounts of alpha-methylstyrene may be and often are employed. The rubber which is used may be natural unvulcanized rubber or it may be a rubbery copolymer of butadiene or isoprene or both with styrene or acrylonitrile. The compositions of the described type are used widely in the plastics industry and products made from them find special favor because of their high impact strength as compared with that of polystyrene.

One common use for the compositions here-concerned is in the production of extruded sheet stock from which shaped articles may be made by the well known process of vacuum molding. For this and other extrusion purposes it is conventional for the plastics manufacturer to convert' the composition to the form of dense pellets or granules suitable in size and shape as feed to a wormfed plastics extruder. Depending on the requirements of the apparatus which is to use the granules, these are generally of approximately uniform size in each batch, and the size required is usually in the range from 0.03 inch to 0.25 inch average diameter, though other sizes are made on request. All of these conventional dense granules, regardless of their cubic dimensions, have surface areasof less than 0.10 and usually less than 0.05 square meter per gram.

The common way of making such molding granules, so far as it is known to the applicant, is to extrude the polymer-rubber composition either in the form of multiple strips or filamentary streams of the desired diameter, or as a thin sheet or strip which while still hot and flowable is deposited on a linearly grooved support with grooves of the desired width and spacing, and then, when the filaments or grooved sheets have solidified, cutting them into pellets or granules of nearly the same length as their width or diameter.

When using such granules as feed to a plastics extruder, as when making sheets, rods, filaments or other extruded shapes from them, it has been common experience that there is a limiting rate of extrusion below which sound, bubble-free extruded products are made but above which the extruded products are defective because of the presence therein of voids or bubbles of various sizes. Such defects, obviously, are to be avoided. It is equally Claims.

Patented Dec. 12, 1961 capacity. Any increase in output which still meets the.

commercial requirements for a sound, void-free product is much to be desired.

Accordingly, it is the principal object of this invention to provide granules of the described composition which give void-free extruded products at a higher rate of output than is possible with the granules heretofore available. A related object is to provide a method for making extrusion granules having such improved qualities. A further object is to provide a method whereby void-free extrudates of the described composition may be made at greater than conventional rates. Other and related objects may appear hereinafter.

The present invention, whereby the foregoing and related objects are attained, is based on the discovery that if, in the course of their production and after they have been cooled below their second order transition temperature, but prior to their use in making extruded articles, there is applied to the granule or to the sheet or strand from which it is to be cut a stress such as to'cause the surface to craze, the resulting granules can be made in the usual sizes but with surface areas of 0.20 to 0.60 or more square meter per gram, and that the pellets socharacterized can be extruded to form void-free products at rates which exceed by 7 to 15 percent the highest rates attainable when making void-free products from the conventional pellets with the described smaller surface areas.

In the annexed drawing,

FIG. 1 illustrates one of the new granules which may be made by the method of the invention, greatly eularged;

FIG. 2 is a schematic representation of one embodiment of the method of the invention; and

FIG. 3 is a schematic representation of another embodiment thereof. p 5

The manner in which the surface crazing of the pellets is accomplished is not critical, so long as it results in an increase in surface area to a value of at least 0.20 square meter per gram. Two convenient and operative procedures will be described, both of which accomplish the desired result by the application of a distorting .force. For purpose of the illustrative examples, the composition used to make the pellets was produced by dissolving a synthetic rubbery copolymer (60 percent butadiene 40 percent styrene) in monomeric styrene and polymerizing the latter. The amount of synthetic rubber present in the final composition was near 5 percent.

. The product was a typical high impact polystyrene.

The modified polystyrene, just described, was extruded through a die having a horizontally disposed row of orifices each near 0.125 inch in diameter. Four batches of pellets were made from the same feed supply in the following manners:

(A) The filamentary product was extruded into a bath of water which was at'room temperature. The strands were withdrawn continuously through a single pair of resilient rubber-covered takeaway rolls, after I which the strands were cut to form granules or pellets of posite sides of the strands. The strands were then cut into granules as described in (A). 'Such a procedure is shown in FIG. 2. V I (D) The extrudate was passed into water and formed into granules, as in (A). The granules were then fed to the nip of a pair of highly polished steel pressure rolls, with sufiicient pressure applied to deform the granules of initially circular cross-section to a somewhat flattened shape in which the longer, transverse dimension (width) from 1.5 to. 2 times as great as the shorter dimension (thickness). This procedure is shown in FIG. 3. The granules obtained in methods (C) and (D) had crazed surfaces, while those of methods (A) and (13) did not.

A measurement was made of the surface area of each of the four batches of granules. Those produced by procedures (A) and (B) are conventional, both as to the manner of production and as to surface area.

Each batch of granules was thereafter used in succession as. feed to the same extruder. In each case the extrusion rate was increased gradually 'until the extrudate showed evidence of bubbles or voids, and was then decreased until such voids no longer appeared in the product. The rate of extrusion at this point was determined in each case in' terms of pounds of composition extruded'per hour to form void-free extrudate. The results are tabulated as follows:

r V Max. ex- Pellets Area, MF/g. trusion Rate,

pounds/hr.

In another si'milar series, in which the granules were about 0.06 inch in average diameter, the following results were obtained.

- Added Volatile Matter Max. ex- Method 7 Area, trusion MF/g Rate, Percent Kind pounds/hr None 0. 01 175 0. Ethylbenzene 0. 47 209 0. 5 Isopropylbenzene 0. 16 206 0 None 0. 54 200 The utility of the present invention has been demonstrated not only with highimpact monoalkenylaromatic hydrocarbon polymers containing about 5 percent of a butadiene-styrene synthetic rubber, but also wit-h such compositions containing from 2 to 20 percent of either natural or synthetic rubbery diene polymer. It has been verified as well with granules of the various sizes from 0.03 to 0.25 inch average diameter, commonly used in commercial extrusion operations. Whenever the granules had sufficiently crazed surfaces to have a surface area of at' least 0.20 square meter per gram, a significantly greater extrusion rate was possible with any of the available types of extruders, without producingan extrudate having bubbles or voids, than when granules of the same size and composition were used having surface areas in the conventional range below 0.10 square meter per gram.

It is to be understood that the here-concerned surface-crazed granules are of essentially the same density as the conventional extrusion granules (near 0.90-1.10) and are not to be confused with spongy or highly porous foamed granules which are generally unsatisfactory as feed to extruders and also are commonly characterized by surface areas of several square meters per gram and by densities under 0.6 gram and usually under 0.3

- gram per cubic centimeter.

The measurement of surface area referred to. herein 1 was accomplished by the method described by Brunauer,

Emmettand'Teller, 1 A. C. S. 60,309 (1938). The method gives results which are reproducible within 15 percent, so that a measurement purporting to show a surface area of 0.20 square meter per gram can be taken.

with assurance to represent a measurable area of from 0.17 to 0.23 square meter per gram.

This application is a division of application Serial No.p69l,913 filed October 23, 1957.

I claim:

1.Tl1e method which comprises extruding a thin hot strip having a density from 0.9 to 1.1 of a composition consisting essentially of a minor proportion, from 2 to 20 percent by Weight, of a rubbery diene polymeric material and a predominant proportion, correspondingly from 98 to percent by weight, of a monoalkenyl benzene series hydrocarbon polymer, cooling the thin strip below its second order transition temperature, subjecting the so-cooled strip to a distorting force sufficient tQ'craze-crack the surface so as to provide thereon when in granular form a surface area of at least 0.20 square meter per gram, and cutting the strip into granules of a size suitable as feed to an extruder.

2. The method claimed in claim 1, wherein the rubbery constituent of the composition employed is a butadiene-styrene copolymer and the predominant polymer constituent is polystyrene.

3. The method claimed in claim 2, wherein the proportion ofthe rubbery copolymer is from 4 to 8 percent.

4. The method claimed in claim 1, wherein the distorting force which craze-cracks the surface is applied by passing the cooled thin strip through a sinuous path References Cited in the file of this patent UNITED STATES PATENTS,

2,391,638 Meakin' .'Dec. 25, 1945 2,464,746 Gering Mar. 15, 1949 2,606,163 Morris et al. Aug. 5, 1952 2,975,483 Cooper et al. Mar. 21, 1961 

1. THE METHOD WHICH COMPRISES EXTRUDING A THIN HOT STRIP HAVING A DENSITY FROM 0.9 TO 1.1 OF A COMPOSITION CONSISTING ESSENTIALLY OF A MINOR PROPORTION, FROM 2 TO 20 PERCENT BY WEIGHT, OF A RUBBERY DIENE POLYMERIC MATERIAL AND A PREDOMINANT PROPORTION, CORRESPONDINGLY FROM 98 TO 80 PERCENT BY WEIGHT, OF A MONOALKENYL BENZENE SERIES HYDROCARBON POLYMER, COOLING THE THIN STRIP BELOW ITS SECOND ORDER TRANSITION TEMPERATURE, SUBJECTING THE SO-COOLED STRIP TO A DISTORTING FORCE SUFFICIENT TO CRAZE-CRACK THE SURFACE SO AS TO PROVIDE THEREON WHEN IN GRANULAR FORM A SURFACE AREA OF AT LEAST 0.20 SQUARE METER PER GRAM, AND CUTTING THE STRIP INTO GRANULES OF A SIZE SUITABLE AS FEED TO AN EXTRUDER. 